In many cases, high carbon steel sheets used for machine structural parts and the like are formed into various shapes and, thereafter, are subjected to a heat treatment for enhancing hardness. Among them, a high carbon steel sheet which contains 0.2 to 0.5 percent by mass of C and in which carbides (cementite) have been subjected to spheroidizing is relatively mild, has excellent formability and, therefore, has been used frequently as a raw material of sheet metal for automobile driving system parts and the like. As for automobile driving system parts which are mass-produced, induction hardening having excellent productivity is suitable for a heat treatment to enhance hardness of parts after forming. Consequently, a high carbon steel sheet having excellent induction hardenability in addition to the formability has been required as a raw material for automobile driving system parts, and various technologies have been proposed previously.
For example, Patent Literature 1 discloses a high carbon steel sheet which is made from hypo-eutectoid steel containing C: 0.1 to 0.8 percent by mass and S: 0.01 percent by mass or less and which has excellent local ductility and hardenability, wherein cementite is dispersed in ferrite in such a way that the spheroidizing ratio becomes 90% or more and the cementite average grain size is 0.4 to 1.0 μm.
Also, Patent Literature 2 proposes a method for manufacturing a high-hardenability high-carbon hot rolled steel sheet by subjecting a steel containing 0.2 to 0.7 percent by mass of C to hot rolling to control the microstructure in such a way as to include more than 20% of bainitic phase on a volume ratio basis and, thereafter, performing annealing in such a way that cementite is spheroidized in the microstructure.
Furthermore, Patent Literature 3 discloses a high carbon steel sheet having excellent hardenability and stretch flangeability, characterized by being made from a steel containing C: 0.22 to 0.45 percent by mass, Cr: 0.01 to 0.70 percent by mass, Ti: 0.005 to 0.050 percent by mass, and B: 0.0003 to 0.0050 percent by mass, wherein the cementite average grain size is 0.1 to 1.0 μm and the ratio of cementite grain size standard deviation/cementite average grain size is 1.0 or less.